It doesn't hurt to have a conversation about it when you are not at work. I wouldn't contribute my input to the supervisor unless it starts to effect me. Somewhere down the long they formed a friendship and it became more than just co-worker vs. co-worker. If he has been spoken to more than once... make your conversation with him a brief one... basically letting him vent about the situation and give him some friendly advice. NOT AT WORK THOUGH. That's just a suggestion if you do choose to say something to him.
If you don't then... just be there for him when his ass gets fired. LOL <3
K I gonna try and go back to sleep. :/ Ciao.